The present invention relates to a method of producing steel strip and the cast steel strip produced according to the method.
In particular, the present invention relates to producing steel strip in a continuous strip caster.
The term xe2x80x9cstripxe2x80x9d as used in the specification is to be understood to mean a product of 5 mm thickness or less.
The applicant has carried out extensive research and development work in the field of casting steel strip in a continuous strip caster in the form of a twin roll caster.
In general terms, casting steel strip continuously in a twin roll caster involves introducing molten steel between a pair of contra-rotated horizontal casting rolls which are internally water cooled so that metal shells solidify on the moving rolls surfaces and are brought together at the nip between them to produce a solidified strip delivered downwardly from the nip between the rolls, the term xe2x80x9cnipxe2x80x9d being used to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed. The casting of steel strip in twin roll casters of this kind is for example described in U.S. Pat. Nos. 5,184,668, 5,277,243 and 5,934,359.
Steel strip is produced of a given composition that has a wide range of microstructures, and therefore a wide range of yield strengths, by continuously casting the strip and thereafter selectively cooling the strip to transform austenite to ferrite in a temperature range between 850xc2x0 C. and 400xc2x0 C. It is understood that the transformation range is within the range between 850xc2x0 C. and 400xc2x0 C. and not that entire temperature range. The precise transformation temperature range will vary with the chemistry of the steel composition and processing characteristics.
Specifically, from work carried out on low carbon steel, including low carbon steel that has been silicon/manganese killed or aluminum killed, it has been determined that selecting cooling rates in the range of 0.01xc2x0 C./sec to greater than 100xc2x0 C./sec to transform the strip from austenite to ferrite in a temperature range between 850xc2x0 C. and 400xc2x0 C., can produce steel strip that has yield strengths that range from 200 MPa to 700 MPa or greater. This is a significant development since, unlike conventional slab casting/hot rolling processes where chemistry changes are necessary to produce a broad range of properties, it has been determined that the same outcome can be achieved with a single chemistry.
Accordingly, there is provided a method of producing steel strip which comprises the steps of:
(a) continuously casting molten low carbon steel into a strip of no more than 5 mm thickness with coarse austenite grains of 100-300 micron width; and
(b) cooling the strip to transform the austenite grains to ferrite in a temperature range between 850xc2x0 C. and 400xc2x0 C. at a selected cooling rate of at least 0.01xc2x0 C./sec to produce a microstructure that provides a strip having a yield strength from between 200 MPa to in excess of 700 MPa, the microstructure selected from a group that includes microstructures that are:
(i) predominantly polygonal ferrite;
(ii) a mixture of polygonal ferrite and low temperature transformation products; and
(iii) predominantly low temperature transformation products.
The term xe2x80x9clow temperature transformation productsxe2x80x9d includes Widmanstatten ferrite, acicular ferrite, bainite and martensite.
The method may include passing the strip onto a run-out table and step (b) includes controlling cooling of the strip on the run-out table to achieve the selected cooling rate to transform the strip from austenite to ferrite in a temperature range between 850xc2x0 C. and 400xc2x0 C.
The method may include the additional step of in-line hot rolling the cast strip prior to cooling the strip to transform the austenite grains to ferrite in a temperature range between 850xc2x0 C. and 400xc2x0 C. This inline hot rolling step reduces the strip thickness up to 15%.
The cast strip produced in step (a) illustratively has a thickness of no more than 2 mm.
The coarse austenite grains produced in step (a) of 100-300 micron width have a length dependent on the thickness of the cast strip. Generally, the coarse austenite grains are up to slightly less than one-half the thickness of the strip. For example, for cast strip of 2 mm thickness, the coarse austenite grains will be up to about 750 microns in length.
The cast strip produced in step (a) may have austenite grains that are columnar.
The upper limit of the cooling rate in step (b) is at least 100xc2x0 C./sec.
The term xe2x80x9clow carbon steelxe2x80x9d is understood to be mean steel of the following composition, in weight percent:
C: 0.02-0.08
Si: 0.5 or less;
Mn: 1.0 or less;
residual/incidental impurities: 1.0 or less; and
Fe: balance
The term xe2x80x9cresidual/incidental impuritiesxe2x80x9d covers levels of elements, such as copper, tin, zinc, nickel, chromium, and molybdenum, that may be present in relatively small amounts, not as a consequence of specific additions of these elements but as a consequence of standard steel making. By way of example, the elements may be present as a result of using scrap steel to produce low carbon steel.
The low carbon steel may be silicon/manganese killed and may have the following composition by weight:
The low carbon steel may be calcium treated aluminum killed and may have the following composition by weight:
The aluminum killed steel may be calcium treated.
The yield strength of aluminum killed steel is generally 20 to 50 MPa lower than that of silicon/manganese killed steel.
Illustratively, the cooling rate in step (b) is less than 1xc2x0 C./sec to produce a microstructure that is predominantly polygonal ferrite and has a yield strength less than 250 MPa.
Illustratively, the cooling rate in step (b) is in the range of 1-15xc2x0 C./sec to produce a microstructure that is a mixture of polygonal ferrite, Widmanstatten ferrite and acicular ferrite and has a yield strength in the range of 250-300 MPa.
Illustratively, the cooling rate in step (b) is in the range of 15-100xc2x0 C./sec to produce a microstructure that is a mixture of polygonal ferrite, bainite and martensite and has a yield strength in the range of 300-450 MPa.
Illustratively, the cooling rate in step (b) is at least 100xc2x0 C./sec to produce a microstructure that is a mixture of polygonal ferrite, bainite and martensite and has a yield strength at least 450 MPa.
The continuous caster may be a twin roll caster.
There is provided a low carbon steel produced by the method described above having desired microstructure and yield strength.